Aqueous, alkaline, cyanide-free bath for the galvanic deposition of zinc alloy coatings

ABSTRACT

An aqueous, alkaline, cyanide-free electrolyte bath for deposition of zinc alloy layers on substrate surfaces is described, which contains cationic pyridinium compounds as brighteners and polyamines as complexing agents. The electrolyte bath is suitable for electroplating bright and even zinc alloy coatings.

FIELD OF THE INVENTION

The present invention relates to an aqueous, alkaline galvanic bath without addition of cyanide ions as complexing agents for the deposition of zinc alloy coatings, in particular, zinc/nickel, zinc/iron and zinc/cobalt coatings which contains, as additives, cationic, heteroaromatic nitrogen compounds, as well as to a process for the deposition of bright and smooth zinc alloy coatings, wherein the bath is used.

BACKGROUND OF THE INVENTION

Zinc deposition from cyanide-containing alkaline solutions has been dominating the industrial market for many years. However, increasingly stringent statutory requirements regarding the disposal of used zinc and zinc alloy electrolyte baths and the consequent strict control regarding waste water have resulted in increased interest in cyanide-free zinc and zinc alloy baths. In this connection, zinc alloy electrolytes are of particular interest.

Such metal coatings are used for improving corrosion properties and for achieving certain optical properties.

Thus, the automotive industry has used electroplated zinc for decades in order to provide highly corrosion-resistant coatings at reasonable cost. However, in view of demands for higher quality and comprehensive warranties, car manufacturers as well as their suppliers have had to develop new coatings systems. In this context, zinc/nickel alloy coatings exhibit the best overall performance with respect to comprehensive corrosion resistance.

It has been found that the amount of nickel in such electroplated zinc/nickel coatings which is useful for improved protection against corrosion, is 4 to 18% of nickel, an amount of 12 to 15% being optimal.

Both, acidic as well as alkaline zinc/nickel baths are known. A review of electroplating baths for industrial application is provided by “Modern Electroplating” (Ed. M. Schlesinger and M. Paunovic) 4th edition (2000), John Wiley & Sons, p. 423-460.

Acidic zinc/nickel alloy baths are typically based on inorganic zinc and nickel salts such as zinc sulfate, zinc chloride, nickel sulfate or nickel chloride, and contain various additives, which improve brightness and grain structure and which also control the zinc/nickel ratio.

Acidic baths for zinc/nickel alloy deposition generally contain acid, such as boric acid of sulfuric acid, and other additives, such as brightener, wetting agents etc.

A disadvantage of such acidic baths is the strong corrosive effect which the electrolyte has on the electroplating equipment. In practice, alkaline baths have been adopted since they provide better metal distribution and, thereby, protect the substrate better against corrosion. Moreover, the incorporation of nickel across a wide range of current densities is substantially more uniform, which also results in improved protection against corrosion. However, these baths exhibit lower cathodic current efficiencies of between 20 and 50%. Such a zinc/nickel bath is described in U.S. Pat. No. 3,681,211, which teaches the use of polyethyleneimine in order to obtain a black coating. According to this document, an aqueous zinc/nickel solution is adjusted to a pH of between 10 and 13 by means of aqueous NaOH solution. The use of polyethyleneimine results in the complexation of the zinc ions and, in particular, the nickel ions which are present in the solution, for example as zinc hydroxide and nickel hydroxide precipitates.

Since the 1970s, polyethyleneimines have been widely used in connection with alkaline zinc and zinc alloy baths. Polyethyleneimines which may be used for this purpose are described in U.S. Pat. No. 3,881,211, according to which they have an average molecular weight of 600 to 100,000 and, in many cases, a complex and branched structure.

U.S. Pat. No. 3,993,548 also describes the use of a high molecular weight polyethyleneimine and of quaternary ammonium silicates for electroplating bright homogeneous zinc layers. The bath described in this document may contain additional brighteners, such as benzyl betaine nicotinate.

Complexing agents other than polyethyleneimines are known as well. U.S. Pat. No. 4,861,442 describes aqueous alkaline baths comprising zinc and nickel ions, alkaline metal hydroxide, an amino alcohol polymer, a complexing agent for nickel and an amino acid and/or a salt of an amino acid. The pH of the bath is 11 or higher.

U.S. Pat. No. 4,877,496 describes aqueous alkaline baths comprising zinc and nickel ions, an alkaline metal hydroxide, a metal complexing agent, a primary brightener and a booster brightener. The primary brightener is a condensation product of an amine, such as ethylenediamine, with epihalodrin. The booster brightener is at least one aromatic aldehyde. Tertiary brighteners such as tellurium oxide, tellurous acid or telluric acid or their salts can also be contained in the baths.

U.S. Pat. No. 4,889,602 describes aqueous electrolyte baths having a pH of more than 11 and comprising zinc and nickel ions as well as at least one compound from the series of aliphatic amines, polymeric aliphatic amines and hydroxy aliphatic carboxylic acids and their salts.

U.S. Pat. No. 5,417,840 describes the use of a bath system primarily containing polyalkyleneimines and complexing agents and pyridinium betaines, in particular, sulfobetaines. DE 198 48 467 describes the use of triethlyenetetramine, tetraethylenepentamine or pentaethylenetetramine as complexing system in combination with N-benzyl nicotinate betaine as primary brightener.

Benzyl pyridinium compounds have been known for some time as brighteners for the deposition of zinc layers. The influence of 3-substituted pyridinium compounds on the texture of electrodeposited zinc layers is described in “Kinzoku Hyomen Gijutsu” (1980), 31, p. 244-248. U.S. Pat. No. 6,652,728 describes the use of N-benzyl-pyridinium-3-carboxylate and N,N′-p-xylene-bis-(pyridinium-3-carboxylate) as brighteners in combination with cationic polymeric urea derivatives for alkaline zinc and zinc alloy baths, in particular zinc iron baths.

All baths for metal deposition known in the prior art exhibit the problem that a uniform layer or a uniform brightness is frequently not achieved across the entire range of current densities. Often zinc/nickel layers having a nickel content of more than 15% are obtained. Moreover, commercially available galvanic baths for depositing zinc/nickel alloy layers often show only moderate cathodic current efficiency resulting in strong evolution of hydrogen.

DESCRIPTION OF THE INVENTION

Therefore, it is the object of the invention to provide a galvanic bath for zinc alloy deposition which produces visually pleasant zinc alloy layers. Additionally, a homogenous zinc/alloy metal distribution and an optimal zinc/alloy metal ratio are to be obtained. Furthermore, a uniform layer thickness, high brightness and uniformity of the alloy components in the coating are to be maintained across a wide range of current densities.

The invention provides an aqueous, alkaline, cyanide-free electrolyte bath for deposition of zinc alloy layers on substrate surfaces, comprising the following components:

a) a source of zinc ions and a source for further metal ions; b) hydroxide ions; c) at least one pyridinium compound of the following formula I or II

wherein R₁ represents a substituted or unsubstituted, saturated or unsaturated, aliphatic or araliphatic hydrocarbon residue having 1 to 12 carbon atoms, R₁′ represents a divalent, substituted or unsubstituted, saturated or unsaturated, aliphatic or araliphatic hydrocarbon residue having 1 to 12 carbon atoms, X₁ represents NR_(x)R_(y) and X₂ represents hydroxyl, OR_(x) or NR_(x)R_(y), wherein R_(x) and R_(y) may be the same or different and represent hydrogen or linear and/or branched alkyl groups having 1 to 12 carbon atoms, and Y⁻ is a counter ion; and d) at least one complexing agent of the general formula III or IV:

wherein X₁, X₂ and X₃ may be the same or different and represent an alkoxy group or a primary, secondary or tertiary amino group, R1, R2 and R3 may be the same or different and represent hydrogen, a C₁-C₁₂ alkyl, C₁-C₁₂ alkoxy, allyl, propargyl or benzyl group, and n and m may be the same or different and represent an integer of 0 to 5.

According to a preferred embodiment, R1 in formula I represents substituted aryl of the following formulae R1a to R1I:

wherein FG represents a residue selected from the group consisting of carboxy, ester, sulfonic acid, carbamoyl, amino, cyano, alkyl, alkoxy, hydroxy, trifluoromethyl, allyl, propargyl-, 4-sulfobutyl, 3-sulfopropyl, 4-carboxybutyl, 3-carboxypropyl residues, hydrogen and halogens, selected from fluorine, chlorine and bromine, and R₁′ represents but-2-enyl, but-2-ynyl or aryl of the following formulae R1′ a to R1′r:

wherein FG represents a residue selected from the group consisting of carboxy, ester, sulfonic acid, carbamoyl, amino, cyano, alkyl, alkoxy, trifluoromethyl residues, hydrogen and halogens, selected from fluorine, chlorine and bromine, wherein all rings or individual fused rings may be substituted.

Preferably, residues R₁ and R₁′ in formulae I and II are bonded to the pyridinium residue via a methylene group.

Preferred araliphatic hydrocarbon residues are, for example, benzyl (R1a) and naphthyl methyl (R1b).

In the context of the present invention, fluorides, chlorides and bromides may be used as halides. Furthermore, the bath according to the invention may contain compounds, which are similar, with respect to their physical and chemical properties, to the halides, i.e., so-called pseudo-halides. Such pseudo-halides are known to the skilled person as such and are described, for example, in Ruml ömpp-Lexikon, Chemie, 10^(th) edition, page 3609. In the context of the present invention, pseudo-halides also comprise residues such as mesylate and triflate.

Preferred counter ions (Y⁻) are halides (for example, fluoride, chloride, bromide) and pseudo-halides.

Preferably, the galvanic bath contains further components in order to improve the properties of the deposited alloy. These may be, for example, polymers of aliphatic amines and metal complexing agents.

The galvanic baths according to the invention contain an inorganic, alkaline component, preferably a hydroxide of the alkali metals, and more preferably sodium hydroxide and/or potassium hydroxide in order to adjust the pH of the bath to at least 10 and preferably to at least 11. The amount of the alkaline component may be 50 to about 250 g/l and preferably 90 to about 130 g/l.

The electrolyte baths according to the invention generally contain zinc ions at a concentration of about 1 to about 100 g/l, concentrations of 4 to 30 g/l being preferred. The zinc ions may be contained in the bath according to the invention in the form of a soluble salt, for example zinc oxide, zinc sulfate, zinc carbonate, zinc acetate, zinc sulfamate, zinc hydroxide, zinc tartrate etc.

As alloying metal, the baths according to the invention contain about 0.1 to 50 g/l of metal ions, suitable metal salts are hydroxides, sulfates, carbonates, ammonium sulfates, sulfamates, acetates, formates and halides, preferably chloride and bromide.

Preferably, the baths according to the invention contain about 0.1 to 50 g/l of nickel ions as alloying metal. Suitable nickel salts are nickel hydroxide, nickel sulfate, nickel carbonate, ammonium nickel sulfate, nickel sulfamate, nickel acetate, nickel formate and nickel halides.

The sources of zinc and alloying metal which may be used in the electrolyte baths according to the invention can comprise one or more of the nickel and zinc sources described above.

The electrolyte baths according to the invention contain the aromatic heterocyclic nitrogen-containing compounds of general formula I or II described above for substantial improvement of leveling and brightness properties of the deposited layers across a wide range of current densities. Accordingly, the compounds of formulae I and II are hereinafter referred to as brighteners according to the invention.

Preferred compounds of formula I or II are 1-benzyl-3-carbamoyl-pyridinium-chloride, 1-(2′-chloro-benzyl)-3-carbamoyl-pyridinium-chloride, 1-(2′-fluoro-benzyl)-3-carbamoyl-pyridinium-chloride, 1-(2′-methoxy-benzyl)-3-carbamoyl-pyridinium-chloride, 1-(2′-carboxy-benzyl)-3-carbamoyl-pyridinium-chloride, 1-(2′-carbamoyl-benzyl)-3-carbamoyl-pyridinium-chloride, 1-(3′-chloro-benzyl)-3-carbamoyl-pyridinium-chloride, 1-(3′-fluoro-benzyl)-3-carbamoyl-pyridinium-chloride, 1-(3′-methoxy-benzyl)-3-carbamoyl-pyridinium-chloride, 1-(3′-carboxy-benzyl)-3-carbamoyl-pyridinium-chloride, 1-(3′-carbamoyl-benzyl)-3-carbamoyl-pyridinium-chloride, 1-(4′-chloro-benzyl)-3-carbamoyl-pyridinium-chloride, 1-(4′-fluoro-benzyl)-3-carbamoyl-pyridinium-chloride, 1-(4′-methoxy-benzyl)-3-carbamoyl-pyridinium-chloride, 1-(4′-carboxy-benzyl)-3-carbamoyl-pyridinium-chloride, 1-(4′-carbamoyl-benzyl)-3-carbamoyl-pyridinium-chloride, (1′-methyl-naphthyl)-3-carbamoyl-pyridinium-chloride, 1-(1′ methyl-naphthyl)-3-carbamoyl-pyridinium-bromide, 1-(1′-methyl-naphthyl)-3-carbamoyl-pyridinium-fluoride, 1,1′-(but-2-enyl)-3,3′-bis-carbamoyl-bispyridinium-dichloride, 1,1′-(but-2-enyl)-3,3′-bis-carboxy-bispyridinium-dichloride, 1-allyl-3-carbamoyl-pyridinium-chloride, 1-allyl-3-carboxy-pyridinium-chloride, 1-propargyl-3-carbamoyl-pyridinium-chloride, 1,1′-(but-2-inyl)-3,3′-bis-carbamoyl-bispyridinium-dichloride, 1,1′-(but-2-inyl)-3,3′-bis-carboxy-bispyridinium-dichloride, 1,1′-(xylenyl)-3,3′-bis-carbamoyl-bis-pyridinium-dibromide, 1-(3′-sulfopropyl)-3-carbamoyl-pyridinium-betain as well as the corresponding bromides, fluorides, iodides and pseudo-halides (for example, triflates, tosylates) of the aforementioned compounds.

The brighteners can be readily prepared by reacting the corresponding nicotinic acid amides or nicotinic acid derivatives with the corresponding benzyl halides in a suitable solvent, such as ethanol, propanol, iso-propanol, butanol, iso-butanol, methanol or their mixtures, DMF, DMAc, NMP, NEP, in substance or in an aqueous medium, while heating under normal or increased pressure. The reaction times required range from 1 to 48 hours, depending on the starting material used. For this purpose, apart from conventional sources of heat, a microwave oven may be used. The pyridinium compounds obtained can either be used directly as the aqueous or alcoholic reaction solution or they can be isolated after cooling by filtration or by removal of the solvent. The compounds can be purified by re-crystallization from a suitable solvent such as ethanol.

Bis-pyridinium compounds of general formula II can be prepared according to U.S. Pat. No. 6,652,728.

The compounds of formula I or II can be used alone or as a mixture at a concentration of 0.001 to 20 WI, preferably of 0.01 to 10 g/l. The baths may contain a combination of pyridinium compounds of formula I or II.

The baths for zinc/nickel deposition according to the invention contain compounds of general formula III or IV as complexing agents. The baths may contain a combination of complexing agents of formula III or IV.

Depending on the zinc and nickel ion concentration, the amounts of polyamine compounds of formula III or IV used in the baths according to the invention lie between 5 and 100 g/l.

Examples of suitable complexing agents for the baths according to the invention are diethylenetriamine, tetraethylenepentamine, pentaethylenehexamine. Moreover, it is possible to use complexing agents as described in U.S. Pat. No. 5,417,840.

The baths according to the invention may be prepared according to conventional methods, for example by adding the specific amounts of the aforementioned components to water. The amount of the basic component, such as sodium hydroxide, should be sufficient to achieve the desired pH of at least 10, preferably above 11.

In a preferred embodiment, the electrolyte bath according to the invention comprises 1 to 100 g/l zinc ions, 0.1 to 50 g/l alloying metal ions, 5 to 100 g/l of at least one compound of general formula III or IV and 0.001 to 20 g/l of at least one compound of general formula I or II or a combination thereof.

In a further preferred embodiment, the electrolyte bath according to the invention comprises 4 to 30 g/l zinc ions, alloying metal ions selected from nickel, iron, cobalt, manganese ions and 0.01 to 10 g/l of at least one compound of general formula I or II or a combination thereof.

In a further preferred embodiment, the electrolyte bath according to the invention contains nickel as alloying metal in an amount of 0.1 to 50 WI, iron in an amount of 10 to 120 mg/l, manganese in an amount of 10 to 100 g/l and/or cobalt in an amount of 10 to 120 mg/l.

The baths according to the invention deposit a bright, even and ductile zinc alloy layer at any conventional temperature of about 15° C. to 50° C., preferably 20° C. to 30° C., more preferably about 25° C. At these temperatures, the baths according to the invention are stable and effective over a wide range of current densities of 0.01 to 10 A/dm², preferably 0.5 to 4 A/dm².

The baths according to the invention can be operated continuously or intermittently and the components will have to be replaced from time to time. The components of the bath can be added alone or in combination. Moreover, depending on the type and the properties of the zinc alloy baths to which the substances are added, they may be varied across a wide range.

Table 1 shows, according to a preferred embodiment with nickel as alloying metal, the influence with respect to layer thickness and nickel incorporation in the electrolytes according to the invention for deposition of zinc/nickel alloy (using 4.03·10⁻⁴ mol/l of the specified pyridinium compound and tetraethylenepentamine as compound of formula III):

TABLE 1 Influence of pyridinium compounds on zinc/nickel alloy depositions (amount used 4.03 · 10⁻⁴ mol/l): Layer Layer thickness thickness Ni conc. Ni conc. Ex. Compound Brightness [μm] [μm] [%] [%] Current density [A/dm²] 3 0.5 3 0.5 1 1-benzyl-3-carbamoyl- ++ 6.5 2.9 14.8 12.7 pyridinium-chloride  2* 1-benzyl-3-carboxy- + 5.8 2.4 15.6 13.5 pyridinium-chloride 3 1,1′-(xylenyl)-3,3′-bis- +++ 7.4 2.5 15.3 13.4 carboxy-bispyridinium- dibromide 4 1,1′-(xylenyl)-3,3′-bis- +++ 7.1 2.2 16.5 13.3 carbamoyl-bispyridinium- dichloride 5 1-(4′-fluoro-benzyl)-3- +++ 6.8 3.5 14.6 13.6 carbamoyl-pyridinium- chloride 6 1-(4′-methoxy-benzyl)-3- ++ 7.2 2 14.4 12.0 carbamoyl-pyridinium- chloride 7 1-(1′-methyl-naphthyl)-3- ++ 8.0 3.5 15.8 13 carbamoyl-pyridinium- chloride 8 1-(4′-carboxy-benzyl)-3- +++ 6 2 14.7 13.0 carbamoyl-pyridinium- chloride 9 1,1′-(but-2-enyl)-3,3′-bis- ++ 7.7 2.0 15.0 14.3 carbamoyl-bispyridinium- dichloride 10  1-allyl-3-carbamoyl- ++ 7.3 2.0 14.4 13.7 pyridinium-chloride *Comparative Example according to DE 198 48 467 C2

As may be taken from Table 1, suitable values with respect to the layer thickness, which is directly proportional to the current efficiency, and with respect to the rate of nickel incorporation, are obtained when using carbamoyl compounds (Examples 1 and 4 to 10) and compounds of formula II (Example 3). Thus, when using the brighteners according to the invention, the current efficiency obtained is up to 38% higher than with conventional brighteners such as 1-benzyl-3-carboxy-pyridinium-chloride (Example 2—Comparative Example), which is mentioned, for example, in DE 102 23 622 B4.

Table 2 shows the influence of the brighteners according to the invention when using standard masses:

TABLE 2 Influence of pyridinium compounds on zinc/nickel alloy depositions (amount used 100 mg/l): Layer Layer Ni Ni thickness thickness conc. conc. Ex. Compound Brightness [μm] [μm] [%] [%] Current density [A/dm²] 3 0.5 3 0.5  1 1-benzyl-3-carbamoyl-pyridinium- ++ 6.5 2.9 14.8 12.7 chloride  2* 1-benzyl-3-carboxy-pyridinium- + 5.8 2.4 15.6 13.5 chloride 11 1,1′-(xylenyl)-3,3′-bis-carboxy- ++ 7.4 2.2 15.0 12.0 bispyridinium-dibromide 12 1,1′-(xylenyl)-3,3′-bis-carbamoyl- ++ 7.5 2.2 14.5 12.7 bispyridinium-dichloride 13 1-(4′-methoxy-benzyl)-3- ++ 8.1 2.7 15.0 13.5 carbamoyl-pyridinium-chloride 14 1-(1′-methyl-naphthyl)-3- ++ 7.5 2.4 14.8 12.5 carbamoyl-pyridinium-chloride 15 1-(4′-carboxy-benzyl)-3-carbamoyl- +++ 8.3 2.6 14.7 12.8 pyridinium-chloride 16 1-(3-sulfopropyl)-3-carbamoyl- ++ 6.2 2.2 14.0 12.2 pyridinium-betaine (300 mg/l) 17 1-benzyl-3-carbamoyl-pyridinium- ++ 7.1 2.7 14.1 12.8 chloride (20 mg/l)/1-(4′-fluoro- benzyl)-3-carbamoyl-pyridinium- chloride (80 mg/l) *Comparative Example according to DE 198 48 467 C2

By varying the groups on the nitrogen and depending on the aryl or alkyl residue used and the groups thereon, different rates of nickel incorporation may be achieved. Moreover, as Tables 1 and 2 show, depending on the brightener used and the amount thereof, different effects with respect to brightness, layer thickness and rate of nickel incorporation may be obtained across a wide range of (molar) amounts and the baths according to the invention may be adjusted by selection of the additive or an additive mixture depending on the desired layer performance.

The combination of the pyridinium compounds used as brighteners and the complexing agents of the baths according to the invention are decisive in this context. Thus, as Table 3 shows, it was surprisingly found that the additive combination as described for alkaline zinc depositions in U.S. Pat. No. 4,071,418 is entirely unsuitable for zinc alloy depositions, in particular zinc/nickel depositions, although the same pyridinium compounds are used. When using the electrolytes as described in the aforementioned patent, only zinc layers are deposited, at substantially lower current efficiencies, although the zinc/nickel complex solution is homogenous.

TABLE 3 Comparison of the zinc/nickel electrolytes according to the invention with a zinc/nickel electrolyte containing the additive combination described in U.S. Pat. No. 4,071,418: Layer Layer Ni Ni thickness thickness conc. conc. Example Additive composition Brightness [μm] [μm] [%] [%] Current density [A/dm²] 3 0.5 3 0.5 1 Bath according to the invention ++ 6.5 2.9 14.8 12.7 18 1-benzyl-3-carbamoyl- + 2.3 1.6 0 0 pyridinium-chloride* *Condensation product of dimethyl amino propylamine and 1,3-dichloropropanol as Comparative Example according to U.S. Pat. No. 4,071,418

A further advantage of the zinc/nickel alloy baths according to the invention is that the use of aromatic aldehydes and tellurite as brighteners is usually unnecessary.

An advantage of the electrolytes of the invention compared with electrolytes according to DE 198 48 467 C2 is the surprisingly low consumption of quaternized nicotinamide derivatives according to the invention compared to N-benzyl nicotinate. As Application Example 19 shows, the consumption of the pyridinium compounds acting as brighteners in the electrolytes according to the invention is significantly lower and thereby more economical than with conventional pyridinium derivatives based on nicotinic acid.

The baths according to the invention may contain, apart from the aforementioned additives, leveling agents such as 3-mecrcapto-1,2,4-triazole and/or thiourea, thiourea usually being preferred.

Surprisingly, it was found that, with the electrolytes according to the invention, the conventional use of aromatic aldehydes or their bisulfate adducts as additional brighteners, for example 4-hydroxybenzylaldehyde, 4-hydroxy-3-methoxybenzaldehyde, 3,4-dimethoxybenzaldehyde, 3,4-methylenedioxybenzaldehyde, 2-hydroxybenzaldehyde or mixtures thereof, is unnecessary.

In a preferred embodiment, the electrolyte bath according to the invention thus contains no aromatic aldehydes or their bisulfate adducts as additional brighteners, in particular, it contains no 4-hydroxybenzylaldehyde, 4-hydroxy-3-methoxybenzaldehyde, 3,4-dimethoxybenzaldehyde, 3,4-methylenedioxybenzaldehyde or 2-hydroxybenzaldehyde or mixtures thereof.

The aqueous, alkaline baths according to the invention can generally be used for all types of substrates on which zinc alloys can be deposited. Examples of suitable substrates are soft steel, spring steel, chromium steel, chromium-molybdenum steel, copper, copper/zinc alloys.

Therefore, the invention also provides a process for galvanic deposition of zinc alloy coatings on conventional substrates, wherein the electrolyte bath according to the invention is used. In this process, the substrate to be coated is immersed into the electrolyte bath.

In the process according to the invention, the deposition of the coatings is preferably carried out at a current density of 0.01 A/dm² to 10 A/dm² and at a temperature in the range of 15 to 50° C., preferably 20 to 30° C., more preferably about 25° C.

The process according to the invention may be carried out, for example, as barrel electroplating process when applied to small pieces and as a rack electroplating process when applied to larger pieces. The process involves the use of anodes, which may be soluble, for example, zinc anodes, which may serve as zinc ion source at the same time so that the zinc deposited on the cathode is replaced by dissolution of zinc from the anode.

However, insoluble anodes (for example, platinized titanium mixed oxide anodes) may also be used, in which case the zinc ions and/or further metal ions removed from the electrolyte by deposition of the alloy have to be added to the electrolyte in other ways, for example by using a zinc dissolution container.

As generally possible in electroplating, the process according to the invention, too, can be carried out with injection of air, with or without agitation of the substrate, which has no negative effects on the coatings obtained. In order to avoid or reduce the oxidation of additives, the electrode regions may be separated or membrane anodes may be used.

The current source may be a conventional rectifier or pulse rectifier.

EXAMPLES

The following examples illustrate the invention, but the invention is not limited thereto.

Preparation Example 1 Synthesis of 1-(4′-methoxy-benzyl)-3-carbamoyl-pyridinium-chloride

In a 100 ml round-bottom flask with reflux condenser, 60 ml of water, 9.2 g of nicotinic acid amide (98%) (0.0738 mol), 11.68 g of 4-methoxybenzylchloride (99%) (0.07378 mol) are heated under reflux for 24 hours. After completion of the reaction, the water is removed in vacuo and the residue is taken up in 200 ml of ethanol and heated under reflux for another hour. The reaction mixture is then cooled to 4° C. and the white solid obtained is removed by filtration and dried in vacuo. This yielded 16.92 g of a white solid (82.26% of the theoretical yield).

Preparation Example 2 Synthesis of 1-(4′-chloro-benzyl)-3-carbamoyl-pyridinium-chloride

In a 100 ml round-bottom flask with reflux condenser, 60 ml of ethanol, 10 g of nicotinic acid amide (98%) (0.0802 mol), 13.05 g of 4-chloro-benzylchloride (99%) (0.0802 mol) are heated under reflux for 24 hours. After completion of the reaction, the solid residue is heated in an ethanol/methanol mixture for another 15 minutes and then cooled to 4° C. The solid obtained is removed by filtration and dried in vacuo. This yielded 18.82 g of a white solid (82.87% of the theoretical yield).

Preparation Example 3 Synthesis of 1-(4′-carboxy-benzyl)-3-carbamoyl-pyridinium-chloride

In a 100 ml round-bottom flask with reflux condenser, 60 ml of ethanol, 7.09 g of nicotinic acid amide (98%) (0.0569 mol), 10.22 g of 4-chloro-benzoic acid (95%) (0.0569 mol) are heated under reflux for 24 hours. After completion of the reaction, the reaction mixture is cooled to 4° C., the resulting solid is removed by filtration and dried in vacuo. This yielded 13.21 g of a white solid (79.21% of the theoretical yield).

Preparation Example 4 Synthesis of 1-(1′-methyl-naphthyl)-3-carbamoyl-pyridinium-chloride

In a 100 ml round-bottom flask with reflux condenser, 60 ml of ethanol, 10 g of nicotinic acid amide (98%) (0.0802 mol), 15.75 g of 4-chloro-methyl-naphthalene (90%) (0.0802 mol) are heated under reflux for 24 hours. After completion of the reaction, the solid residue is heated in 200 ml of an ethanol/methanol mixture (75:25) for another 15 minutes and then cooled to 4° C. The resulting solid is removed by filtration and dried in vacuo. This yielded 19.37 g of a white solid (80.84% of the theoretical yield).

Preparation Example 5 Synthesis of 1-(4′-fluoro-benzyl)-3-carbamoyl-pyridinium-chloride

In a 100 ml round-bottom flask with reflux condenser, 60 ml of ethanol, 10 g of nicotinic acid amide (98%) (0.0802 mol), 11.72 g of 4-fluoro-benzyl-chloride (99%) (0.0802 mol) are heated under reflux for 24 hours. After completion of the reaction, the solid residue is heated in 200 ml of an ethanol/methanol mixture for another 15 minutes and then cooled to 4° C. The resulting solid is removed by filtration and dried in vacuo. This yielded 18.13 g of a white solid (84.76% of the theoretical yield).

Preparation Example 6 Synthesis of 1,1′-(xylenyl)-3,3′-bis-carbamoyl-bis-pyridinium-dichloride

In a 100 ml round-bottom flask with reflux condenser, 60 ml of ethanol, 10 g of nicotinic acid amide (98%) (0.0802 mol), 7.16 g of α,α′-dichloro-p-xylene (98%) (0.0401 mol) are heated under reflux for 24 hours. After completion of the reaction, the solid residue is heated in 200 ml of an ethanol/methanol mixture for another 15 minutes and then cooled to 4° C. The resulting solid is removed by filtration and dried in vacuo. This yielded 12.29 g of a white solid (73.13% of the theoretical yield).

Preparation Example 7 Synthesis of 1-allyl-3-carbamoyl-pyridinium-chloride

In a 100 ml round-bottom flask with reflux condenser, 60 ml of ethanol, 10 g of nicotinic acid amide (98%) (0.0802 mol), 6.26 g of allyl-chloride (98%) (0.0802 mol) are heated under reflux for 24 hours. After completion of the reaction, the reaction mixture is cooled to 4° C. and the resulting solid is removed by filtration and dried in vacuo. This yielded 9.25 g of a white solid (58.07% of the theoretical yield).

Preparation Example 8 Synthesis of benzyl-3-carbamoyl-pyridinium-chloride

In a 100 ml round-bottom flask with reflux condenser, 60 ml of ethanol, 10 g of quinoline (98%) (0.0802 mol), 10.252 g of benzyl chloride (99%) (0.0802 mol) are heated under reflux for 24 hours. After completion of the reaction, the reaction mixture is cooled to 4° C. and the resulting solid is removed by filtration and re-crystallized from 1 liter of ethanol. This yielded 19.00 g of a white solid (95.33% of the theoretical yield).

Preparation Example 9 Synthesis of 1,1′-(but-2-enyl)-3,3′-bis-carbamoyl-bis-pyridinium-dichloride

In a 100 ml round-bottom flask with reflux condenser, 60 ml of ethanol, 10 g of nicotinic acid amide (98%) (0.0802 mol), 5.90 g (0.0401 mol) of trans-1,4-dichloro-2-butene (85%) are heated under reflux for 24 hours. After completion of the reaction, the reaction mixture is cooled to 4° C. and the resulting solid is removed by filtration and dried in vacuo. This yielded 13.64 g of a white solid (92.19% of the theoretical yield).

Application Example 1

An electrolyte having the following composition is used:

  12 g/l Zn(OH)₂ 9.55 g/l NiSO₄•6H₂O  120 g/l NaOH   36 g/l tetraethylenepentamine  100 mg/l 1-benzyl-3-carbamoyl-pyridinium-chloride

250 ml of the electrolyte are filled into a Hull cell. A nickel anode is used. The cathode is electroplated at 1A for 15 minutes. After completion of the electroplating, the metal sheet is rinsed and dried with compressed air. The layer thickness is measured at two points (3 cm from the bottom edge and 2.5 cm from the right- and left-hand edge) at high (3 A/dm²) and low current density (0.5 A/dm²). The measurement of the nickel content is carried out at the same places. The measurement is done by XRF and four points in each position so as to minimize measurement errors. The coating obtained was highly bright.

The following layer thicknesses and nickel contents were obtained:

Current density Layer thickness Ni conc. [A/dm²] [μm] [%] 3.0 6.5 14.8 0.5 2.9 12.7

Application Example 2 Comparative Example According to DE 102 23 622 A1

Application Example 1 is repeated with the exception that an electrolyte having the following composition is used:

  12 g/l Zn(OH)₂ 9.55 g/l NiSO₄•6H₂O  120 g/l NaOH   36 g/l tetraethylenepentamine  100 mg/l 1-benzyl-3-carboxy-pyridinium-chloride

A bright coating having the following layer thicknesses and nickel contents was obtained:

Current density Layer thickness Ni conc. [A/dm²] [μm] [%] 3.0 5.8 15.6 0.5 2.4 13.5

Application Example 3

Application Example 1 is repeated with the exception that an electrolyte having the following composition is used:

  12 g/l Zn(OH)₂ 9.55 g/l NiSO₄•6H₂O  120 g/l NaOH   36 g/l tetraethylenepentamine  166 mg/l 1,1′-(xylenyl)-3,3′-bis-carboxy-bis-pyridinium-dibromide

A very bright coating having the following layer thicknesses and nickel contents was obtained:

Current density Layer thickness Ni conc. [A/dm²] [μm] [%] 3.0 7.4 15.3 0.5 2.5 13.4

Application Example 4

Application Example 1 is repeated with the exception that an electrolyte having the following composition is used:

  12 g/l Zn(OH)₂  9.55 g/l NiSO₄•6H₂O   120 g/l NaOH   36 g/l tetraethylenepentamine 163.4 mg/l 1,1′-(xylenyl)-3,3′-bis-carboxy-bis-pyridinium-dichloride

A very bright coating having the following layer thicknesses and nickel contents was obtained:

Current density Layer thickness Ni conc. [A/dm²] [μm] [%] 3.0 7.1 16.5 0.5 2.2 13.3

Application Example 5

Application Example 1 is repeated with the exception that an electrolyte having the following composition is used:

12 g/l Zn(OH)₂ 9.55 g/l NiSO₄•6H₂O 120 g/l NaOH 36 g/l tetraethylenepentamine 108 mg/l 1-(4′-fluoro-benzyl)-3-carbamoyl-pyridinium-chloride

A very bright coating having the following layer thicknesses and nickel contents was obtained:

Current density [A/dm²] Layer thickness [μm] Ni conc. [%] 3.0 6.8 14.6 0.5 3.5 13.6

Application Example 6

Application Example 1 is repeated with the exception that an electrolyte having the following composition is used:

12 g/l Zn(OH)₂ 9.55 g/l NiSO₄•6H₂O 120 g/l NaOH 36 g/l tetraethylenepentamine 88 mg/l 1-(4′-methoxy-benzyl)-3-carbamoyl-pyridinium-chloride

A very bright coating having the following layer thicknesses and nickel contents was obtained:

Current density [A/dm²] Layer thickness [μm] Ni conc. [%] 3.0 7.2 14.4 0.5 2.0 12.0

Application Example 7

Application Example 1 is repeated with the exception that an electrolyte having the following composition is used:

12 g/l Zn(OH)₂ 9.55 g/l NiSO₄•6H₂O 120 g/l NaOH 36 g/l tetraethylenepentamine 123.5 mg/l 1-(1′-methyl-naphthyl)-3-carbamoyl-pyridinium-chloride

A very bright coating having the following layer thicknesses and nickel contents was obtained:

Current density [A/dm²] Layer thickness [μm] Ni conc. [%] 3.0 8.0 15.8 0.5 3.5 13.0

Application Example 8

Application Example 1 is repeated with the exception that an electrolyte having the following composition is used:

12 g/l Zn(OH)₂ 9.55 g/l NiSO₄•6H₂O 120 g/l NaOH 36 g/l tetraethylenepentamine 120.6 mg/l 1-(4′-carboxy-benzyl)-3-carbamoyl-pyridinium-chloride

A very bright coating having the following layer thicknesses and nickel contents was obtained:

Current density [A/dm²] Layer thickness [μm] Ni conc. [%] 3.0 6.0 14.7 0.5 2.0 13.0

Application Example 9

Application Example 1 is repeated with the exception that an electrolyte having the following composition is used:

12 g/l Zn(OH)₂ 9.55 g/l NiSO₄•6H₂O 120 g/l NaOH 36 g/l tetraethylenepentamine 139.9 mg/l 1,1′-(but-2-enyl)-3,3′-bis-carbamoyl-pyridinium-dichloride

A very bright coating having the following layer thicknesses and nickel contents was obtained:

Current density [A/dm²] Layer thickness [μm] Ni conc. [%] 3.0 7.7 15.0 0.5 2.0 14.3

Application Example 10

Application Example 1 is repeated with the exception that an electrolyte having the following composition is used:

12 g/l Zn(OH)₂ 9.55 g/l NiSO₄•6H₂O 120 g/l NaOH 36 g/l tetraethylenepentamine 76.85 mg/l 1-allyl-3-carbamoyl-pyridinium-chloride

A very bright coating having the following layer thicknesses and nickel contents was obtained:

Current density [A/dm²] Layer thickness [μm] Ni conc. [%] 3.0 7.3 14.4 0.5 2.0 13.7

Application Example 11

Application Example 1 is repeated with the exception that an electrolyte having the following composition is used:

12 g/l Zn(OH)₂ 9.55 g/l NiSO₄•6H₂O 120 g/l NaOH 36 g/l tetraethylenepentamine 100 mg/l 1,1′-(xylenyl)-3,3′-bis-carboxy-bis-pyridinium-dibromide

A very bright coating having the following layer thicknesses and nickel contents was obtained:

Current density [A/dm²] Layer thickness [μm] Ni conc. [%] 3.0 7.4 15.0 0.5 2.2 12.0

Application Example 12

Application Example 1 is repeated with the exception that an electrolyte having the following composition is used:

12 g/l Zn(OH)₂ 9.55 g/l NiSO₄•6H₂O 120 g/l NaOH 36 g/l tetraethylenepentamine 163.4 mg/l 1,1′-(xylenyl)-3,3′-bis-carbamoyl-bis-pyridinium-dichloride

A very bright coating having the following layer thicknesses and nickel contents was obtained:

Current density Layer thickness Ni conc. [A/dm²] [μm] [%] 3.0 7.5 14.5 0.5 2.2 12.7

Application Example 13

Application Example 1 is repeated with the exception that an electrolyte having the following composition is used:

  12 g/l Zn(OH)₂ 9.55 g/l NiSO₄•6H₂O  120 g/l NaOH   36 g/l tetraethylenepentamine   88 mg/l 1-(4′-methoxy-benzyl)-3-carbamoyl-pyridinium-chloride

A very bright coating having the following layer thicknesses and nickel contents was obtained:

Current density Layer thickness Ni conc. [A/dm²] [μm] [%] 3.0 8.1 15.0 0.5 2.7 13.5

Application Example 14

Application Example 1 is repeated with the exception that an electrolyte having the following composition is used:

  12 g/l Zn(OH)₂ 9.55 g/l NiSO₄•6H₂O  120 g/l NaOH   36 g/l tetraethylenepentamine  100 mg/l 1-(1′-methyl-naphthyl)-3-carbamoyl-pyridinium-chloride

A very bright coating having the following layer thicknesses and nickel contents was obtained:

Current density Layer thickness Ni conc. [A/dm²] [μm] [%] 3.0 7.5 14.8 0.5 2.4 12.5

Application Example 15

Application Example 1 is repeated with the exception that an electrolyte having the following composition is used:

  12 g/l Zn(OH)₂ 9.55 g/l NiSO₄•6H₂O  120 g/l NaOH   36 g/l tetraethylenepentamine  100 mg/l 1-(4′-carboxy-benzyl)-3-carbamoyl-pyridinium-chloride

A very bright coating having the following layer thicknesses and nickel contents was obtained:

Current density Layer thickness Ni conc. [A/dm²] [μm] [%] 3.0 8.3 14.7 0.5 2.6 12.8

Application Example 16

Application Example 1 is repeated with the exception that an electrolyte having the following composition is used:

  12 g/l Zn(OH)₂ 9.55 g/l NiSO₄•6H₂O  120 g/l NaOH   36 g/l tetraethylenepentamine  300 mg/l 1-(3′-sulfo-propyl)-3-carbamoyl-pyridinium-betain

A very bright coating having the following layer thicknesses and nickel contents was obtained:

Current density Layer thickness Ni conc. [A/dm²] [μm] [%] 3.0 6.2 14.0 0.5 2.2 12.2

Application Example 17

Application Example 1 is repeated with the exception that an electrolyte having the following composition is used:

  12 g/l Zn(OH)₂ 9.55 g/l NiSO₄•6H₂O  120 g/l NaOH   36 g/l tetraethylenepentamine   80 mg/l 1-(4′-fluoro-benzyl)-3-carbamoyl-pyridinium-chloride

A very bright coating having the following layer thicknesses and nickel contents was obtained:

Current density Layer thickness Ni conc. [A/dm²] [μm] [%] 3.0 7.1 14.1 0.5 2.7 12.8

Application Example 18 Comparative Example According U.S. Pat. No. 4,071,418

Application Example 1 is repeated with the exception that an electrolyte having the following composition is used:

  12 g/l Zn(OH)₂ 9.55 g/l NiSO₄•6H₂O  120 g/l NaOH   76 ml 10% solution of the condensation product of dimethyl amino propylamine and 1,3-dichloropropanol prepared according to Example 1 of U.S. Pat. No. 4,071,418  100 mg/l 1-benzyl-3-carbamoyl-pyridinium-chloride

A bright, brownish gleaming coating having the following layer thicknesses and nickel contents was obtained:

Current density Layer thickness Ni conc. [A/dm²] [μm] [%] 3.0 2.3 0 0.5 1.6 0

Application Example 19 Long-Term Experiment with a 5 Liter Bath for Determining the Additive Consumption

In a comparative experiment, electrolytes having the following composition are used:

  12 g/l Zn(OH)₂ 9.55 g/l NiSO₄•6H₂O  120 g/l NaOH   36 g/l tetraethylenepentamine  100 mg/l a pyridinium compound (N-benzylnicotinate or 1-benzyl-3- carbamoyl-pyridinium-chloride)

The use of N-benzylnicotinate corresponds to the teaching of DE 198 48 467 C2.

In order to compare the consumption of additives of the electrolyte according to the invention and the electrolyte according to DE 198 48 467 C2, both electrolytes are used in a 5 liter bath as in the Application Examples described above in order to electroplate Norton sheets. In this experiment, a Norton sheet is electroplated at 6 A for 30 minutes, whereafter the layer thickness and the visual appearance are evaluated. When the zinc and nickel contents were sufficient and the visual appearance of the Norton sheets was good and bright, the electroplating is continued. At intervals of 50 Ah (10 Ah/l), a complete bath test is carried out, consisting of the Hull cell test (as described above) and the determination of the zinc and NaOH concentration. If too little zinc or nickel (target value: 10 g/l zinc oxide; 2 g/l nickel) or NaOH is present, the missing amount is added. After the brightness has decreased, the corresponding pyridinium compound is replenished. Table 4 shows the additive consumption of pyridinium compounds used as brighteners relative to 10,000 Ah.

TABLE 4 Application Example 19 Consumption/ Entry Pyridinium compound 10 kAh Percent consumption 1 N-benzylnicotinate 62.5 g 100% (reference) 2 1-benzyl-3-carbamoyl-   10 g 16% pyridinium-chloride 

1. Aqueous, alkaline, cyanide-free electrolyte bath for deposition of zinc alloy layers on substrate surfaces, comprising the following components: a) a source of zinc ions and a source for further metal ions; b) hydroxide ions; c) at least one pyridinium compound of the general formula I or II:

wherein R₁ represents a substituted or unsubstituted, saturated or unsaturated, aliphatic or araliphatic hydrocarbon residue having 1 to 12 carbon atoms, R₁′ represents a divalent, substituted or unsubstituted, saturated or unsaturated, aliphatic or araliphatic hydrocarbon residue having 1 to 12 carbon atoms, X₁ represents NR_(x)R_(y) and X₂ represents NR_(x)R_(y), wherein R_(x) and R_(y) may be the same or different and represent hydrogen or linear and/or branched alkyl groups having 1 to 12 carbon atoms, and Y⁻ is a counter ion; and d) at least one complexing agent of the general formula III or IV:

wherein X₁, X₂ and X₃ may be the same or different and represent an alkoxy group or a primary, secondary or tertiary amino group, R1, R2 and R3 may be the same or different and represent hydrogen, a C1-C12 alkyl, C1-C12 alkoxy, allyl, propargyl or benzyl group, and n and m may be the same or different and represent an integer of 0 to
 5. 2. Electrolyte bath according to claim 1, wherein R₁ represents substituted aryl of the following formulae R₁a to R₁I:

wherein FG represents a residue selected from the group consisting of carboxy, ester, sulfonic acid, carbamoyl, amino, cyano, alkyl, alkoxy, hydroxy, trifluoromethyl, allyl, propargyl-, 4-sulfobutyl, 3-sulfopropyl, 4-carboxybutyl, 3-carboxypropyl residues, hydrogen and halogens, selected from fluorine, chlorine and bromine, and R₁′ represents but-2-enyl, but-2-ynyl or aryl of the following formulae R1′ a to R1′ r:

wherein FG represents a residue selected from the group consisting of carboxy, ester, sulfonic acid, carbamoyl, amino, cyano, alkyl, alkoxy, trifluoromethyl residues, hydrogen and halogens, selected from fluorine, chlorine and bromine, wherein all rings or individual fused rings may be substituted.
 3. Electrolyte bath according to one of claim 1, containing a combination of pyridinium compounds of formulae I or II.
 4. Electrolyte bath according to claim 1, containing a combination of complexing agents of formulae III or IV.
 5. Electrolyte bath according to claim 1, comprising 1 to 100 g/l of zinc ions, 0.1 to 50 g/l alloying metal ions, 5 to 100 g/l of at least one compound of general formula III or IV and 0.001 to 20 g/l of at least one compound of general formula I or II or a combination thereof.
 6. Electrolyte bath according to claim 5, comprising: 4 to 30 g/l of zinc ions, alloying metal ions selected from nickel, iron, cobalt, manganese ions and 0.01 to 10 g/l of at least one compound of general formula I or II or a combination thereof.
 7. Electrolyte bath according to claim 1, containing as alloying metal nickel in an amount of 0.1 to 50 g/l, iron in an amount of 10 to 120 mg/l, manganese in an amount of 10 to 100 g/l and/or cobalt in an amount of 10 to 120 mg/l.
 8. Electrolyte bath according to claim 1, containing as base an alkali metal hydroxide.
 9. Electrolyte bath according to claim 8, wherein the alkali metal hydroxide is sodium hydroxide or potassium hydroxide and is present in an amount of 50 to 250 g/l.
 10. Electrolyte bath according claim 1 having a pH of at least
 10. 11. Process for galvanic deposition of bright and even zinc alloy coatings, comprising a step of immersing the substrate to be coated into a bath according to claim
 1. 12. Process according to claim 11, wherein the bath is operated at a current density of 0.01 to 10 A/dm².
 13. Process according to claim 11, wherein the bath is operated at a temperature of 15 to 50° C.
 14. Process according to claim 11, wherein the bath is operated at a temperature of about 25° C.
 15. Process according to claim 11, wherein the coatings are applied to a conductive substrate by using a drum electroplating process.
 16. Process according to claim 11, wherein the coatings are applied to a conductive substrate using a rack electroplating process.
 17. Process according to claim 1, wherein a coating of a zinc alloy with one or more metals from the group consisting of cobalt, nickel, manganese and iron is deposited on the substrate. 